A quick grep for GetDocument() shows that there are really only a few places where we don't check to see if the result is valid:

probe::didSuspendAudioContext(GetDocument());
probe::didResumeAudioContext(GetDocument());
probe::didCloseAudioContext(GetDocument());

All the remaining uses are related to AutoPlay.  I have no idea if the autoplay methods support a null Document.

So assign to hongchan to look at the probe issue, and have mlamouri look at the autoplay uses.

Methods for probing are a part of JS interface calls. (AudioContext's resume, suspend, close) So there is no way for the document to go away when this call happens.

In other words, there is no cross-thread activity here.

Assigning to mlamouri to take a look if this matters for the autoplay stuff.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/724697cd3cc1d5e248e9054fb1896bef878f17f7

commit 724697cd3cc1d5e248e9054fb1896bef878f17f7
Author: Mounir Lamouri <mlamouri@chromium.org>
Date: Thu Sep 20 15:53:01 2018

WebAudio: some checks around GetDocument() calls.

Adding a DCHECK() and a null check.

Bug:  883329 
Change-Id: I56efb844c7cc6788ac04e9f494608ef18e94cd78
Reviewed-on: https://chromium-review.googlesource.com/1236454
Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592817}
[modify] https://crrev.com/724697cd3cc1d5e248e9054fb1896bef878f17f7/third_party/blink/renderer/modules/webaudio/audio_context.cc